In the mining of coal in the United States, specialized mining machines perform the act of digging the coal from coal seams. Due to the crushing effect encountered in the mining and preparation of coal, a large amount of fines (fine particles of coal) of a size less than 28 mesh are generated. For many, many years a great percentage of these fines were discarded as waste.
Since 1900, an estimated 50,000,000 tons of coal fines have been discarded into pond structures in the coal producing states of Pennsylvania, Maryland, West Virginia, Virginia, Kentucky, Tennessee, Ohio and Indiana. Some discarding of the ultra fines continues to date.
Prior attempts to separate coal fines have included froth flotation which is a process originated in the 1950's. Froth flotation has an efficiency rating of approximately 50% to 60% and is a very high cost method because of the electric power and chemicals required in this process.
Flotation is a widely used separation process which is based on the fact that some components in comminuted minerals are wettable (hydrophilic), whereas others are water-repellent (hydrophobic). Hydrophobic particles have an ability to hold air bubbles by surface action, the nature of the film on the outside of the particles being the controlling factor. Finely divided air which is introduced into the "pulp", the mixture of solids and water in which flotation is performed, adheres in the form of bubbles to these particles, more particularly the metalliferous components of the pulp, and causes them to rise to the surface. Here they collect in a mass of froth and are removed by a skimmer device. The hydrophilic components remain behind in the pulp. As a rule, these are the worthless minerals (gangue), which are removed as tailings from the flotation machine.
The floatability of minerals can be controlled by certain chemical additives called flotation agents. These are of various kinds:
Frothers, whose function is to produce froth by combining the air bubbles (introduced into the pulp by stirring or by the injection of compressed air) into a stable froth which will buoy up the ore particles. Oils and allied substances are used as frothers.
Collectors are substances that increase the water repellency and make the ore particularly receptive to the attachment of air bubbles. Collectors usually consist of synthetic organic compounds.
Other flotation agents help to regulate the process. So-called depressors can make hydrophobic minerals temporarily hydrophilic and can in this way help in the selective separation of one mineral from another by depressing one, thereby inhibiting its flotation. The "depressed" mineral can subsequently be made hydrophobic again by an activating agent. The various agents for regulating the flotation process in this manner are inorganic compounds, mostly salts.
One attempt to separate fine coal particles is disclosed in U.S. Pat. No. 4,203,831 to Parnaby. This patent describes a coal washing plant having two cyclone separators for separating coal from dross.
Other separators for separating coarse from fine particles are disclosed in U.S. Pat. No. 3,366,247 to Visman, U.S. Pat. No. 3,887,456 to Loughner, U.S. Pat. No. 3,940,331 to Rastatter, U.S. Pat. No. 4,123,364 to Mozley, U.S. Pat. No. 4,226,708 to McCartney, U.S. Pat. No. 4,308,134 to Lilleker et al., U.S. Pat. No. 4,670,161 to Hayatdavoudi, U.S. Pat. No. 4,711,720 to Young, U.S. Pat. No. 4,793,925 to Duvall et al., U.S. Pat. No. 4,859,347 to Simon et al., U.S. Pat. No. 4,865,740 to Callut, and U.S. Pat. No. 5,078,549 to Schweiss et al.